- 1 - EUROPEAN SOUTHERN OBSERVATORY Organisation Européenne pour des Recherches Astronomiques dans l Hémisphère Austral Europäische Organisation für astronomische Forschung in der südlichen Hemisphäre OBSERVING PROGRAMMES OFFICE Karl-Schwarzschild-Straße 2 D-85748 Garching bei München e-mail: opo@eso.org Tel. : +49-89-32 00 64 73 APPLICATION FOR OBSERVING TIME PERIOD: 87A Important Notice: By submitting this proposal, the PI takes full responsibility for the content of the proposal, in particular with regard to the names of CoIs and the agreement to act according to the ESO policy and regulations, should observing time be granted 1. Title Category: B 5 DATING THE ACQUISITION EVENTS IN GALAXIES WITH COUNTER-ROTATING STELLAR DISKS 2. Abstract / Total Time Requested Total Amount of Time: 0 nights VM, 17 hours SM Unveiling the fate of gas and stars resulting from merging events is crucial to understand galaxy evolution. Recently, we have observed with VIMOS/IFU the interacting spiral NGC 5719. It hosts a massive stellar counterrotation and is characterized by a stream of counter-rotating gas stripped from the nearby companion NGC 5713. We have successfully separated the two stellar conter-rotating components by measuring their line-strength radial profiles. The counter-rotating stellar component formed less than 2 Gyr ago. It is therefore likely they have formed after the newly acquired counter-rotating gas. We propose now to repeat these successful observations targeting the 2 other disk galaxies hosting counter-rotating stars and visible from Paranal Observatory. We will measure the age, metallicty, and α-enhancement of the two counter-rotating components and date the last acquisition event in order to characterize the merging process. 3. Run Period Instrument Time Month Moon Seeing Sky Mode Type A 87 VIMOS 17h apr g 1.2 CLR s 4. Number of nights/hours Telescope(s) Amount of time a) already awarded to this project: b) still required to complete this project: 5. Special remarks: 6. Principal Investigator: Alessandro Pizzella, alessandro.pizzella@unipd.it, I, Università di Padova,Dipartimento di Astronomia 6a. Co-investigators: F. Bertola Universita di Padova,Dipartimento di Astronomia,I L.M. Buson INAF - Osservatorio Astronomico di Padova,I L. Coccato Max Planck Institut fuer extraterrestrische Physik,D E.M. Corsini Universita di Padova,Dipartimento di Astronomia,I Following CoIs moved to the end of the document... 7. Is this proposal linked to a PhD thesis preparation? State role of PhD student in this project
8. Description of the proposed programme A Scientific Rationale: The phenomenon of counter-rotation between stars and/or gas has been detected in large number of galaxies, with morphological types ranging the entire Hubble sequence (see Bertola & Corsini 1998, IAU Stymp. 186, 149, for a review). Although some attempts have been made to explain stellar counter-rotation as due to self-induced phenomena like dissolving bars (e.g. Evans & Collett 1994, ApJ, 420, L67), this phenomenon is usually considered as the signature of a past merging events, like the acquisition of gas from the environment or the capture of a satellite in a opposite direction with respect to the motion of the main stellar disk. The particular case of extended stellar counter-rotation (in which the two counter-rotating stellar disks have comparable masses and dimensions) is extremely rare (Pizzella et al. 2004, A&A, 424, 447). It has been observed only in few spiral galaxies: NGC 3593 (Bertola et al. 1996, ApJ, 458, L67; Corsini et al. 1998, A&A, 337, 80), NGC 4138 (Jore et al. 1996, AJ, 112, 438), NGC 4550 (Rubin et al. 1992, ApJ, 394, L9; Rix et al. 1992, ApJ, 400, L5), NGC 5719 (Vergani et al. 2007, A&A, 463, 883), and NGC 7217 (Merrifield & Kuijken 1994, ApJ, 432, 575). In a recent paper (Vergani, Pizzella, Corsini et al. 2007, A&A, 463, 883) we studied the case of NGC 5719, an almost edge-on Sab galaxy with a prominent skewed dust lane. It shows a spectacular on-going interaction with its face-on Sbc companion NGC 5713. The projected distance between the two galaxies is 70 kpc. NGC 5719 hosts two counter-rotating stellar disks, one of them is associated with the gas stripped from NGC 5713. The Galex/NUV and FUV images by Neff et al. (2005, ApJ, 619, 91) show star forming regions within the HI bridge and tails connecting NGC 5719 and NGC 5713. They also reveal the presence of a diffuse 400 Myr-old population in the stellar disk of NGC 5719, but they can not resolve its spatial distribution due to the limited resolution provided by Galex (about 5 ). With the recent proposal 383.B-0632 we obtained VIMOS/IFU spectroscopy of the galaxy. The results we obtained are spectacular (Coccato, Morelli, Pizzella et al., in prep.). We measured the velocity field of the ionised gas, co-rotating and the counter-rotating components (Fig. 1). Due to the different velocity of the two co-spatial stellar components, we could separate the spectra of the two components (Fig.2) and obtain for each of them a measure of the Lick indices demonstrating that the counter-rotating stars are younger and more metal-poor than the co-rotating ones (Fig. 3). The counter-rotating components formed less than 2 Gyr ago. For the first time the age, metallicity, and α-enhancement were derived for two counter-rotating stellar disks, proving the scenario according to which the newly acquired gas settles into disk and forms stars (see Corsini & Bertola 1998, Jour. Korean Phys. Soc., 33, 574). Our strategy was therefore successful both from the technical and scientific aspects. We propose now to take similar data for the two counter-rotating galaxies NGC 3593 and NGC 4550 and to perform the same analysis. These are the only two galaxies hosting extended counter-rotating disks known so far, visible from Paranal Observatory. B Immediate Objective: Aim of the proposal is to apply the same analysis technique applied for NGC 5719 (Coccato et al., in prep.) on two disk galaxies to the age, metallicity, and α-enhancement of their counter-rotating stellar component components to date the epoch of the last acquisition event. NGC 3593. This Sa galaxy has been studied in detail by Bertola et al. (1996). The galaxy is, in some respect, similar to NGC 5719: its ionised gas is counter-rotating with respect to the main stellar component (with 80% of the disk stars) and there is a counter-rotating stellar structure (with the remaining 20% of the disk stars) that seems associated to it. In the same position of the counter-rotating stars there is a CO emission (Garcia-Burillo et al. 2000, A&A, 363, 869) and an Hα ring (Corsini et al. 1998). It is likely that the galaxy has experienced an acquisition event. The acquired gas then formed the counter-rotating stars. However, the galaxy shows no apparent sign of interaction and the nearest neighbourhood is located at 150 kpc. NGC 4550. This S0 galaxies was studied by Rubin et al. (1992) and Rix et al. (1992). They found the presence of two counter-rotating stellar components of the same intensity. The ionised gas was rotating with one of the two components. Emsellem et al. (2004, MNRAS, 352, 721) by means of the SAURON 2D spectrograph found that the component rotating like the ionised gas forms a thin disk structure while the other component is much broader. Kuntschner et al. (2010, MNRAS, in press) measured the Lick indices finding that the counter-rotating thin component is younger than the thick component. However, they did not resolved the two components and their Lick indices measurements suffer from contamination between the two components. We will be able to disentangle kinematics and stellar population properties of both components, simultaneously. Contrary to NGC 5719, NGC 3593 and NGC 4550 are undisturbed galaxies with no evident signature of recent merging or acquisition. Measuring the age of their two counter-rotating stellar disks will allow us to disentangle between the formation scenarios of internal (by bar dissolution) and external (by gas accretion) origin. If the disks formed from orbital scattering due the dissolution of a bar, they are expected to be coeval. Otherwise, if the counter-rotating stellar disk is formed after acquired gas, we will observe that the stellar disk associated to the gaseous disk is younger than the other. Details about the data reduction technique are given in Box9 Time Justification. - 2 -
8. Description of the proposed programme and attachments Fig. 1: Measured radial velocity of the main stellar disk (left panel), counter-rotating stellar disk (central panel), and counter-rotating ionised gas disk (right panel) in NGC 5719. North is right, East is up (Coccato et al., in prep.). Observed and Model Spectrum Counter rotating Component Rotating Component Ionized Gas Fig. 2: Example of observed spectrum (black line) and model spectrum (green line) for one of the fibres. The spectra of the two stellar and the ionised gas components are also shown separately. Fig. 3: Measured Lick indices in the main stellar disk (filled dots) and counter-rotating stellar disk (open squares). The predictions from single stellar population models by Thomas et al. (2003) are superimposed (Coccato et al., in prep.). - 3 -
9. Justification of requested observing time and observing conditions Lunar Phase Justification: According to the ETC dark or gray time can fit our observations Time Justification: (including seeing overhead) Goal of the proposal is to obtain spectra in the 4800-5300Å spectral range (including Hβ, Mg, Fe) to derive simultaneously the detailed stellar kinematics and stellar population properties at each position on the galaxy. At each position in the field of view, the observed galaxy spectrum is decomposed into the contributions from the spectra of two stellar and one ionised-gas components (Fig.2). The spectral decomposition is performed using an implementation of the penalised pixel fitting routine, which allows to disentangle kinematics and stellar population contents of the three components superimposed along the line of sight. To this aim we need spectra with a S/N of at least 30 on each fibre. The faintest surface brightness we need to reach is about 20 mag/ 2. According to the ETC we need about 2 hour and 27 minutes to reach a S/N of 15. We will then rebin 2 2 fibres in the fainter regions to reach the S/N of 30. In each OB we can integrate 49 minutes on the target, we integrate 5 minutes on the sky, and telescope+instrument overheads are 26 minutes, totalising 1h 20m for each OB. For each pointing we plan to execute 3 identical OBs (with some dithering between them in order to deal with dead fibres) reaching a total on-target integration time of 2h and 27m necessary do accomplish our requirements. In both targets the field of view we need to observe is about 20 40. Since the VIMOS-IFU field of view is 27 27 in high-resolution mode, we need 2 pointings for each galaxy. Last, we plan to observe with the same setup three giant stars to be used as Lick templates. These are bright stars that require few seconds of integration. Considering overhead the three stars requires about 1 hour in total Summarising, we need 6 OBs of 1 hours 20 minutes for each galaxy, 1 hour for standards and therefore 17 hours to complete our program. All the above strategy and OBs has been successfully executed in program 383.B-0632 for NGC 5719. 9a. Telescope Justification: VIMOS-IFU in combination with the 0.67 fiber is the ideal instrument to perform this task as it has the proper spatial and spectral resolution. The field of view of 27 27 available with the 0.67 /fiber configurations also perfectly matches our goal. 9b. Observing Mode Justification (visitor or service): Observations are straightforward. Our presence at the telescope is not required. 9c. Calibration Request: Standard Calibration
10. Report on the use of ESO facilities during the last 2 years 383.B-0632. Data fully reduced. Paper near submission. The results are shown in this proposal. 10a. ESO Archive - Are the data requested by this proposal in the ESO Archive (http://archive.eso.org)? If so, explain the need for new data. Are the data requested in this proposal in the ESO Archive (http://archive.eso.org)? The data are not present in the archive. 10b. GTO/Public Survey Duplications: 11. Applicant s publications related to the subject of this application during the last 2 years Coccato, L.; Gerhard, O.; Arnaboldi, M.; 2010 MNRAS 407, L26: Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889. Coccato, L.; Arnaboldi, M.; Gerhard,O.; Freeman,K. C.; Ventimiglia, G.; Yasuda, N.; 2010 A&A 519, 95: Kinematics and line strength indices in the halos of the Coma brightest cluster galaxies NGC 4874 and NGC 4889 Buson, L.M. 2010, A&A, 519, 59: The mid-uv population of the nucleus and the bulk of the post-merger NGC 3610 Cardullo, A.; Corsini, E. M.; Beifiori, A.; Buson, L. M.; Dalla Bontá, E.; Morelli, L.; Pizzella, A.; Bertola, F. 2009, A&A 508, 641 The ultraviolet flare at the center of the elliptical galaxy NGC 4278 Morelli, L.; Cesetti, M.; Corsini, E. M.; Pizzella, A.; Dalla Bont, E.; Sarzi, M.; Bertola, F. 2010, A&A 518, 32: Multiband photometric decomposition of nuclear stellar disks Pizzella, A.; Tamburro, D.; Corsini, E. M.; Bertola, F., 2008, A&A, 482, 53: Detection of non-ordered central gas motions in a sample of four low surface brightness galaxies Morelli, L.; Pompei, E.; Pizzella, A., Corsini, E.M. et al., 2008, MNRAS, 389, 341: Stellar populations of bulges in 14 cluster disc galaxies Pizzella, A.; Corsini, E. M.; Sarzi, M.; et al., 2008, MNRAS, 387, 1099: Structure and dynamics of galaxies with a low surface-brightness disc - I. The stellar and ionized-gas kinematics M. Cesetti, V. D. Ivanov, L. Morelli, A. Pizzella, L. Buson, E. M. Corsini et al. 2008, A&A 497, 41: Early-type galaxies in the near-infrared: 1.5-2.4µm spectroscopy Corsini, E. M.; et al. 2008, ApJS, 175, 462: Spatially Resolved Spectroscopy of Coma Cluster Early-Type Galaxies. IV. Completing the Data Set. - 5 -
12. List of targets proposed in this programme Run Target/Field α(j2000) δ(j2000) ToT Mag. Diam. Additional info Reference star A NGC 3593 11 14 37.0 +12 49 04 8.0 12.6 cz= 628km/s A NGC 4550 12 35 30.6 +12 13 15 8.0 12.3 cz= 381km/s A SAO 159459 15 40 22.5-10 46 18 0.35 7.2 standard A SAO 158946 14 56 11.0-10 56 39 0.32 5.9 standard A SAO 140444 15 15 52.1-00 16 47 0.32 5.9 standard - 6 -
13. Scheduling requirements - 7 -
14. Instrument configuration Period Instrument Run ID Parameter Value or list 87 VIMOS A IFU 0.67 /fibre HR-Blue - 8 -
6b. Co-investigators:...continued from box 6a. L. Morelli Universita di Padova,Dipartimento di Astronomia,I - 9 -